Effect of liquid phase on scale formation during high-temperature oxidation of AlSi-transformation-induced plasticity steel surfaces

The oxidation kinetics, and the structural evolution of the resulting surface scale, of cast transformation-induced plasticity (TRIP) steel (0.97 wt pet Al and 1.11 wt pet Si) has been investigated in the temperature range of 850°C to 1250°C under atmospheres with oxygen partial pressures close to 0.2 atm. Direct visualization using a high-temperature confocal scanning laser microscope (CSLM) showed that at 1050°C and higher temperatures, a liquid oxide phase formed beneath the surface, penetrating and liquefying the scale that had formed initially. After a period of time, which was dependent on temperature, the liquid became fully crystallized. A microprobe analysis of the steel/scale interface indicated an Al₂O₃-SiO ₂-FeO_n composition in the liquid oxide. This phase formed a network that penetrated the scale. The rest of the outer scale consisted primarily of Fe₂O₃, while Al-Si-rich oxides were observed close to the metal/scale interface. Thermogravimetric analysis indicated a parabolic growth rate below 1000°C and a linear growth rate at 1000°C. At higher temperatures, a parabolic rate dominated once again. The scale thickness appears to be limited by the time period during which the liquid oxide could contribute to rapid mass transfer, which resulted in the observed linear oxidation rate. As the upper temperature limit of the linear oxidation region is reached, the liquid oxide becomes enriched with FeO_n, decreasing the stability of the liquid phase. This leads to crystallization of solid Fe oxides at the surface or the formation of appreciable amounts of Al- and Si-rich oxides at the interface. These processes block access of the liquid oxide to the steel.